Following unilateral lumbosacral deafferentation of the cat hindlimb, Substance P reaction product demonstrated by the unlabeled antibody (PAP) technique in the lumbar dorsal horn at first decreases and then partially returns. This pattern of change may be interpreted as indicating sprouting by Substance P containing axons. Interneurons appear to be one source for the returning SP reaction product. In the coming year the ultrastructure of the sprouting terminals will be determined and compared to the SP containing terminals normally present using quantitative morphological techniques. In addition, the changes in SP content which follow deafferentation will be studied by radioimmunoassay (RIA).